1. Technical Field
The present invention relates generally to an apparatus and method for non-destructive testing of mechanical properties within solid materials using the LCR ultrasonic technique. Specifically, the present invention relates to an improved technique and apparatus that provides a more accurate measurement of degradation in polymeric rotational molded components such as pressure vessels, tanks and piping.
2. Description of Related Art
The non-destructive testing for stress in materials has long been recognized as an important method for evaluating structural components to predict both the failure location and rate, to identify stressed components prior to failure, and many other safety related considerations. Non-destructive testing is used extensively in a wide variety of industries including the aviation, automotive, petroleum, and chemical industries, and various construction and structural related fields. The use of non-destructive testing on specific components ranges from the testing of polymeric and metal piping and pressure vessels for loss of wall as well as for fatigue cracks. Additionally, steel turbine blades in jet engines, steel support beams in bridges, and other large structures are monitored by nondestructive testing. Components in use can be tested to determine the stress levels in the components without damaging or destroying the components. The benefit of non-destructive testing is self-evident.
In polymeric materials, mechanical properties may change with time. The changes may be due to environmental effects caused by ultraviolet exposure, the chemical environment of the surrounding air as well as the commodity contained in the tank. Laboratory studies of samples held under UV light for extended periods of time show that there is a correlation between the level of degradation of a rotational molded polyethylene tank and its ultrasonic characteristics, namely velocity and attenuation. This correlation is measurable and quantifiable. Based on these findings, field trials were conducted on a number of in-service rotational molded cross-linked polyethylene tanks at different sites, for both industrial and fertilizer storage applications. These tanks varied in their service life from present year production to more than fifteen years of service. The ultrasonic wave characteristics from the field tasks were in line with the lab observations. These results indicate that the level of overall degradation of the tank may be quantified based on the measured relevant ultrasonic data.
Since the mechanical properties affect the likelihood of failure, a convenient and reliable nondestructive technique for evaluating the mechanical properties of polymer tanks in service would be useful to industry. Ultrasonic properties such as speed, attenuation, and frequency characteristics are directly related to the mechanical properties of the material. A newly designed apparatus using ultrasonic LCR waves excited and received on the outer curved surfaces of rotational molded polymeric tanks will enable the inspection of tanks in the field.